Metal halide discharge lamp having heat absorbing coating

ABSTRACT

The arc tube ends of a metal halide lamp have a double layer coating thereon, the first coating being a heat absorbing coating and the second coating being a heat reflective coating.

United States Patent [1 1 Bamberg et al.

[ METAL HALIDE DISCHARGE LAMP HAVING HEAT ABSORBING COATING [75] Inventors: William I. Bamberg, Medford;

William M. Keeffe, Rockport, both of Mass.

[73] Assignee: GTE Sylvania Incorporated,

Danvers, Mass.

[22] Filed: June 3, I974 [21] Appl. No: 475,733

[52] U.S. Cl. 313/44; 117/215; 117/221;

117/124 A; 313/47; 313/221; 313/220; 313/45 [51] Int. Cl. IIOIJ 61/52 [58] Field of Search 313/45 44, 47, 221, 220;

[ 1 Aug. 19, 1975 [56] References Cited UNITED STATES PATENTS 3.842.304 10/1974 Beyer et al 313/44 Primar ExaminerR. V. Rolinec Assistant Examiner-Darwin R Hostetter Attorney, Agent, or Firm.lames Theodosopoulos [57] ABSTRACT The are tube ends of a metal halide lamp have a double layer coating thereon, the first coating being a heat absorbing coating and the second coating being a heat reflective coating.

6 Claims, 1 Drawing Figure METAL HALIDE DISCHARGE LAMP HAVING HEAT ABSORBING COATING THE INVENTION This invention relates to high intensity metal halide are discharge lamps. Such lamps comprise an arc tube, usually made of fused quartz or other high silica glass, having electrodes disposed at its ends and containing a fill including an inert starting gas, mercury and one or more metal halides. The mercury is completely vaporized during normal lamp operation while the metal halides are usually only partially vaporized.

The are tube press sealed ends of such lamps usually have a heat reflective coating thereon in order to maintain said ends at a sufficiently high temperature so as to ensure adequate vapor pressure of the metal halides in the arc tube. Examples of such coatings are shown in US. Pats. Nos. 3,325,662 and 3,374,377, which disclose coatings of calcium pyrophosphate and zirconium dioxide.

However, in spite of these heat reflective coatings, usually white, the area behind the electrodes is usually the coolest part of the arc tube. This results in condensation of the low vapor pressure metal halides thereat, after lamp extinguishment. Since a crevice usually exists in the quartz glass at the junction of the electrode assembly with the press seal, the metal halides often condense out within the crevice. Once this occurs, there is little chance for the crevice area to reach a high enough temperature to drive these condensed halides back into the arc tube where they would be available to the arc discharge.

High intensity metal halide discharge lamps are generally formulated to include quantities of several different metal halides which, given the operating conditions within the arc tube and vapor pressure characteristics of the halides, will insure the correct partial pressures of the halides in the discharge to cause the arc to radiate with the desired spectral characteristics. If a portion of the metal halide fill is lost into the crevice region, the spectral characteristics of the discharge change. Not only is the total quantity of available metal halide reduced by condensation, but also the chemical balance of the remaining halide fill is changed from optimum, since the different vapor pressure characteristics of each fill component cause crevice condensation out of proportion to the desired balance in the are discharge tube.

A purpose of this invention is to eliminate such crevice condensation and thereby obviate changes in the discharge spectral characteristics resulting therefrom during lamp life.

This is accomplished by first coating the region behind the electrodes, or at least in the immediate area of the crevice formation, with a dark or gray coating. Such a coating is a better heat absorber than the heat reflective coatings of the prior art. Then, in order to prevent heat loss by rcradiation from the first coating, a second heat reflective coating is directly deposited onto the first coating. This second coating can be a white prior art coating.

An embodiment of this invention is shown in FIG. I which is an elevational view of a metal halide are discharge tube. Arc tube 1 has press seals 2 at each end thereof. At each end of are tube 1 are electrodes 3 which are supported by lead-in wires 4. Lead-in wires 4 extend into and are embedded in press seals 2. The previously mentioned crevice exists at the point where lead-in wire 4 enters press seal 2. Lead-in wires 4 are connected to a molybdenum ribbon 5, embedded within press seal 2, to which external lead-in wires 6 are connected. A starting electrode 7, connected to an external lead-in wire 8, is also located at one end of are tube 1.

Coating 9, which covers the ends of are tube 1 behind electrodes 3, extends from about the vicinity of electrodes 3 onto press seal 2. Coating 9 comprises a first coating of a dark or gray material, on top of which a second coating of white heat reflective material is deposited. In one example, this first coating consisted of a mixture of zirconium diboride (ZrB- and zirconium dioxide (ZrO- while the second coating consisted of ZTOZ.

Several 400 watt metal halide lamps having an arc tube coating in accordance with this invention were evaluated and compared with control lamps having only the prior art white reflective coating. All are tubes contained a fill of Si mg mercury, 5.0 mg mercuric iodide, 20 mg sodium iodide, 0.7 mg scandium, 0.5 mg thorium and 27 torr argon. This fill is designed to yield high lamp efficacy with good white light, that is, light having a high color rendering index.

The lamps in accordance with this invention yielded 35,600 lumens initially versus 33,200 for the control lamps. After 6000 hours operation, the lamps of this invention yielded 24,500 lumens, for a lumen maintenance of The control lamps had dropped to 21,300 lumens, a lumen maintenance of only 6l7c.

An analysis of the radiation intensity of the various fill components of the 6000 hour operated lamps was made in order to compare the changes between the two sets of lamps. The analysis was made by comparing the ratio of the intensity of the spectral lines which are characteristic of the particular metals in the fill to the intensity of the 4358 angstrom mercury line. The ratio of the 5896 sodium line to the 4358 mercury line was 0.213 for the lamps of this invention versus 0.l33 for the control lamps. For the 5688 and 5682 sodium lines, said ratios were (H74 and 0.094, respectively, for lamps of this invention, as against 0.055 and 0.028 for the control lamps. The 4499 thorium line had a ratio of 0.2l l for the invention lamps and 0. l 38 for the control lamps. For the 4734 scandium line, the rations were 0.356 and 0.188, respectively. These results show that there was proportionately more sodium-thoriumscandium radiation from the invention lamps, after 6000 hours operation, than there was from the control lamps; thus there was less condensation of the iodides of sodium, thorium and scandium in the invention lamps than there was in the control lamps.

The only spectral line, of those analyzed, which showed a higher ratio in the control lamps than in the invention lamps was the 5770 mercury line. The ratio of the 5770 mercury line to the 4358 mercury line was 0.043 in the invention lamps and 0.062 in the control lamps. The reason for this is that as the various metal halides condense out in the are tube, the radiation characteristics of the arc discharge more closely approach those of a mercury discharge, with its relatively low efiicacy, blue-green radiation and higher are temperature.

In order to determine if the are temperature of the control lamps, after 6000 hours, was higher than the are temperature of the invention lamps, the ratios of ionized to atomic radiation for scandium and thorium were determined. Higher concentration of ions indicates a higher are temperature. The ratio for thorium was determined by comparing the intensity of the 4391 thorium ion line with the 4499 thorium atom line; for scandium, the 4374 scandium ion line was compared with the 4734 scandium atom line.

Said ratio for thorium was 0.632 for the invention lamps and 1.071 for the control lamps. For scandium the ratios were 0.724 and 1.089, respectively. This confirms the higher are temperature characteristic of a mercury discharge lamp, and the increased condensation of the metal halide additives. in the prior art control lamps.

In these lamps, the dark coating consisted of 7r ZrB and 75% ZrO The coating was applied from a suspension prepared by mixing the necessary quantities of a ZrB suspension and a ZrO suspension to yield a suspension containing 25% ZrB and 75% ZrO The ZrB suspension consisted of 50 grams ZrB- 1.25 grams colloidal alumina and lOOO ml isopropyl alcohol. The ZrO- suspension consisted of 2000 grams ZrO 50 grams colloidal alumina and 2500 grams isopropyl alcohol. The coating was applied by dipping each end of sealed arc tube 1 into the dark coating suspension, removing the excess from press seal 2 and then firing the coating, for example, at 550C to 800C, to improve its adhesion. The second coating was applied over the first coating by also dipping the are tube ends into the ZrO suspension, removing the excess from press seal 2 and tiring the coating.

Although this invention is applicable to the use of any suitable heat absorbing material, such as carbon black, for the first layer of coating 9, there is particular advantage to the use of ZrB. or a mixture of ZrB and ZrO- A suitable material has a high melting point and a low vapor pressure at the normal operating temperature of the arc tube, that is to say, it does not emit gas during normal lamp operation.

Under the conditions to which coating 9 is exposed during lamp operation, there is a gradual decomposition of the ZrB a borosilicate glass and zirconium metal are the end product. Borosilicate glass is transparent while the zirconium metal can act as an oxygen getter and fonn ZrO which is the white reflective coating normally used on an arc tube. This reaction takes place while tungsten blackening occurs within the arc tube. With the external coating becoming lighter while a dark inner coating forms, the temperature characteristics affecting condensate are far more stable 0 than in prior art lamps. This results in a discharge which shows very little color change throughout lamp life. This is significant since the human eye is far more sensitive to color variation than it is to absolute intensity. Spectroscopic data have confirmed that lamps made in accordance with this invention and operated for 6000 hours show spectral radiation very similar to a new lamp, with little change in either are temperature or color temperature. Prior art lamps operated for 6000 hours exhibited the characteristic shift toward a pure mercury lamp, with the associated increases in both are temperature and color temperature. This indi eates that lumen losses in the lamps of this invention are almost entirely due to interior are tube wall blackening, while such blackening, together with condensate losses, affect both the output and the color of the radiation from prior art lamps.

The emittance of various mixtures of ZrB and ZrO was measured in the infrared to determine their relative efficiency as heat absorbers. The usual white ZrO coating of the prior art has an emittance of 0.36 (reflectivity of 0.64) while a coating of ZrB has an emittance of 0.96. Mixtures of ZrB -ZrO containing 12 /270, 25%, 50% and ZrB have emittences, respectively, of 0.72, 0.91 0.95 and 0.96. Thus a mixture containing 25% ZrB- is almost as efficient a heat absorber as ZrB. alone.

We claim:

I. An arc discharge lamp comprising an arc tube, made of high silica glass and having press seals at each end, containing a filling including inert starting gas, mercury and metal halide, having electrodes sealed therein at opposite ends and having a coating on the ends of said are tube, said coating comprising a first layer of a dark or gray heat absorbing material and a second layer of a white heat reflecting material. said dark or gray heat absorbing material comprising zirconium diboride.

2. The lamp of claim 1 wherein the first layer comprises a mixture of zirconium diboride and zirconium dioxide.

3. The lamp of claim 3 wherein zirconium diboride comprises at least l2 k7r of said mixture.

4. The lamp of claim 3 wherein zirconium diboride comprises at least 25% of said mixture.

5. The lamp of claim 1 wherein said second layer comprises zirconium dioxide.

6. The lamp of claim I wherein said first layer comprises a mixture of ZrB and ZrO and said second layer comprises ZrO 

1. AN ARC DISCHARGE LAMP COMPRISING AN ARC TUBE, MADE OF HIGH SILICA GLASS AND HAVING PRESS SEALS AT EACH END, CONTAINING A FILLING INCLUDING INERT STARTING GAS, MERCURY AND METAL HALIDE HAVING ELECTRODES SEALED THEREIN AT OPPOSITE ENDS AND HAVING A COATING ON THE ENDS OF SAID ARC TUBE, SAID COATING COMPRISING A FIRST LAYER OF A DARK OR GRAY HEAT ABSORBING MATERIAL AND A SECOND LAYER OF A WHITE HEAT REFLECTING MATERIAL, SAID DARK OR GRAY HEAT ABSORBING MATERIAL COMPRISING ZIRCONIUM DIBORIDE.
 2. The lamp of claim 1 wherein the first layer comprises a mixture of zirconium diboride and zirconium dioxide.
 3. The lamp of claim 3 wherein zirconium diboride comprises at least 12 1/2 % of said mixture.
 4. The lamp of claim 3 wherein zirconium diboride comprises at least 25% of said mixture.
 5. The lamp of claim 1 wherein said second layer comprises zirconium dioxide.
 6. The lamp of claim 1 wherein said first layer comprises a mixture of ZrB2 and ZrO2 and said second layer comprises ZrO2. 